The A10 at Waterbeach has had the limit reduced after a baby died in an accident earlier this year.
Louis Thorold died as he was walking with his mother Rachael in January when a car and van collided, leading the van to mount the pavement where it both people.
Five-month-old Louis died at the scene and his mother suffered multiple injuries.
The road near the Denny End junction to just before the slip road off Ely Road at Milton now has a reduced speed limit of 40mph, down from 50mph, with new speed limit signs and road markings having been introduced by Cambridgeshire County Council.
The introduction of the new limit follows a campaign by the Thorold family including the establishment of the Louis Thorold Foundation to campaign for safer roads and provide support to other victims.
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